Shear-wave propagation in orthorhombic Phenolic CE: A comparison of numerical and physical modelling

Georg Rümpker and R. James Brown

ABSTRACT

In this paper we analyze effects of shear-wave propagation in orthorhombic Phenolic CE (PCE). This industrial laminate provides a physical model for the study of wave propagation in anisotropic media and has been used in a number of laboratory experiments. In a recent experiment by Brown et al. (1993) polarity reversals were found on seismogram traces along two profiles through a sphere of PCE. The observations were attributed to the rapid variation of polarization in the neighbourhood of slowness surface conical points (point singularities). Based on numerical modelling experiments we attempt an interpretation of these observations. The numerical results show amplitude variations similar to those observed in the physical modelling. The amplitude variations for receiver positions along a symmetry plane of the anisotropic medium may indeed be attributed to rapid polarization changes due to conical points. However, the numerical examples indicate that relatively smooth variations of the displacement on crossing a symmetry plane can result in rapid amplitude variations (polarity reversals) on seismogram traces, depending on the transmitter/receiver configuration used. The calculated seismograms also show characteristic (Hilbert-transform type) waveforms due to wavefront folding. This folding is a direct results of slowness surface conical points and the related waveform characteristics may be used in future experiments to detect conical point effects. However, the detectability of these waveform variations depends strongly on the frequency range emitted by the source.

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